The particular proteins and the concentration thereof in body fluids such as ascites fluid, spinal fluid and urine, may indicate a particular pathological condition. For diagnostic purposes, it is desirable to separate the proteins from the fluids to some extent, that is, to obtain a fluid having a substantially higher concentration of the proteins, to facilitate subsequent analysis through procedures such as electrophoresis and gel chromatography. Collodion bags have previously been used for dialyzing the body fluid to obtain the desired concentration but as the amount of the fluid in the bag is reduced during the dialysis procedure, the bag surface area in contact with the fluid decreases. This results in a reduction of the speed of dialysis and reduced efficiency of the bag membrane. Furthermore, presently available means for removal of samples of the concentrated fluid from the collodion bag have certain drawbacks, among which are: the need for a separate pipetting or fluid withdrawal system; the use of narrow bore pipette tips can damage the collodion bag; a potential hazard to the operator if mouth pipetting is employed, a common laboratory practice. Additionally, Pasteur pipettes which are normally employed for such purposes are fragile and can easily be broken off in the apparatus.
For reference purposes, collodion bags are small porosity membranes formed to separate out proteins of different molecular sizes from the fluids in which they reside. Collodion bags may be used to separate out molecules having a size down to as low as 10,000 molecular weight. The bag itself is formed of cellulose nitrate which forms the membrane, the substance of the membrane generally being referred to by the term collodion. The membrane is, in effect, a fine orifice lattice structure used for mechanical filtration of molecules from fluids in which the molecules are carried or suspended.